1. Field of the Invention. The present invention in general relates to level sensors for sensing the level of liquid or other substances in relatively deep tanks, such as underground gasoline storage tanks, and more particularly to such a sensor which is relatively inexpensive and yet is capable of measuring liquid level continuously over a relatively large depth range.
2. The art relating to level sensing is voluminous. Some level sensors sense discrete levels. See for example U.S. Pat. No. 3,783,689 issued to Albert D. Ehrenfried et al; No. 4,382,382 issued to Jish M. Wang; No. 3,955,416 issued to William Paul Waiwood and No. 4,756,076 issued to Jerry F. Dyben et al. These devices generally use resistance type sensors combined with simple integrated circuit electronics. Others measure the level continuously. See for example U.S. Pat. No. 3,100,885 issued to W. Welkowitz et al; No. 4,679,432 issued to Erich Draeger; No. 4,685,332 issued to Joseph T. Betterton et al; No. 4,702,107 issued to Giampaolo Guerrini et al; No. 4,745,293 issued to Douglas A. Christensen; No. 4,747,062 issued to Norman D. Esau, and No. 4,748,846 issued to John S. Haynes. These generally use more complex measuring devices such as ultrasonics, optics, etc. combined with sophisticated electronics. The latter continuous devices are generally more accurate than the former discrete devices. However, the former are generally simpler and less expensive than the latter. It would be highly advantageous to have a level sensor that utilized relatively simpler and less expensive technology of resistance type sensors and simple integrated circuit electronics and at the same time measured the level continuously.
It is known to measure both the level and temperature in a tank with a single probe. See for example U.S. Pat. No. 4,231,025 issued to John H. Turner, Jr. and the co-pending U.S. Pat. Application Ser. No. 07/116,920 by the present inventors. Membrane potentiometers for use as touch-activated switches are also known.